Fiber optic cables are used for a variety of applications. A fundamental problem associated with the use of fiber optic cables is the inherent fragility of glass fibers. This fragility renders it difficult to form a flexible cable that can withstand bending, twisting, impact, vibration, and other types of stress.
Failures in glass fiber elements typically result from surface flaws or imperfections in the fiber elements. The stress concentration at such surface flaws may be many times greater than the nominal stress at the same point. Because there is no axial yielding in glass fibers, stress at surface flaws cannot be relieved. If a high tensile stress is applied to a glass fiber, surface flaws may propagate with eventual failure of the fibers. Thus, it is necessary that the fiber optic elements be protected from crushing forces or other forces which might nick the surfaces of the fiber optic elements to create points of high stress concentration, while at the same time providing a flexible fiber optic cable. The various types of damage (denting, crushing, kinking, twisting, etc.) often occur during installation and use due to the cable being bent over sharp objects, clamped too tightly, struck by another object, twisted, or bent beyond its minimum bend radius.
U.S. Pat. No. 4,147,406 to Anderson provides one approach for solving this problem. In Anderson, a fiber optic cable is provided wherein a spiral-cut, nonmetallic tube is applied over a glass fiber optic core comprised of one or more fiber optic element bundles cabled along an axial strength member having greater tensile strength and lower elongation per unit length than the fiber optic elements. Using this construction, the axial loading to which the cable may be subjected is taken by the axial strength member rather than subjecting the fiber optic bundles to undesirably high tensile forces. A thin-walled, nonmetallic jacket over the spiral-cut, nonmetallic tube and, optionally, a textile braid or serve layer between the cabled fiber optic bundles and the spiral-cut tubing are also provided in Anderson for improved abrasion resistance. Although this device provides some protection for the fiber optic cable, a more crush resistant fiber optic cable than that produced according to Anderson is desirable.
U.S. Pat. No. 5,061,823 to Carroll describes a crush-resistant coaxial transmission line. This patent describes a spiraled, rigid, metal wire layer between the outer conductor of the coaxial transmission line and the outer jacket of the cable. A ruggedized fiber optic cable would be desirable.